Lecture16

Physics 101: Lecture 16 Angular Momentum Today’s lecture will cover Textbook Chapter 8.7-8.9 Exam II

Overview Review K rotation = ½ I  2 Torque = Force that causes rotation Equilibrium  F = 0  = 0 Today Angular Momentum L = I   L = 0 if  = 0

Linear and Angular Linear Angular Displacement x  Velocity v  Acceleration a  Inertia m I KE ½ m v2 ½ I  2 N2L F=ma  = I  Momentum p = mv L = I  Today

Define Angular Momentum Momentum Angular Momentum p = mV L = I  conserved if  F ext = 0 conserved if  ext =0 Vector Vector! units: kg-m/s units: kg-m 2 /s

Right Hand Rule Wrap fingers of right hand around direction of rotation, thumb gives direction of angular momentum. What is direction of angular momentum for wheel A) Up B) Down C) Left D) Right

Act: Two Disks A disk of mass M and radius R rotates around the z axis with angular velocity  i . A second identical disk, initially not rotating, is dropped on top of the first. There is friction between the disks, and eventually they rotate together with angular velocity  f . A)  f =  i B)  f = ½  i C)  f = ¼  i  i Wheel rim drop z  f z

Act: Two Disks First realize that there are no external torques acting on the two-disk system. Angular momentum will be conserved!  0 z 2 1  f z

Lecture 16, Pre-flights You are sitting on a freely rotating bar-stool with your arms stretched out and a heavy glass mug in each hand. Your friend gives you a twist and you start rotating around a vertical axis though the center of the stool. You can assume that the bearing the stool turns on is frictionless, and that there is no net external torque present once you have started spinning. You now pull your arms and hands (and mugs) close to your body.

Bonus Question! There are No External forces acting on the “student+stool” system. A) True B) False C) What!? Key is no external torques about vertical axis! FBD has gravity and normal force.

Lecture 16, Preflight 1 What happens to the angular momentum as you pull in your arms? 1. it increases 2. it decreases 3. it stays the same “ Because there are no external torques present, angular momentum is conserved ” L 1 L 2 CORRECT

Lecture 16, Preflight 2 What happens to your angular velocity as you pull in your arms? 1. it increases 2. it decreases 3. it stays the same “ you decreased the I so w increases ” “ I tried this out Friday night ” “ thanks for the new drinking game”  1  2 I 2 I 1 L L CORRECT

Lecture 16, Preflight 3 What happens to your kinetic energy as you pull in your arms? 1. it increases 2. it decreases 3. it stays the same “ KE is inversly related to intertia, therefore, increases.” CORRECT  1  2 I 2 I 1 L L (using L = I  )

What about Energy Conservation? A) Energy isn’t conserved here B) Energy comes from weights C) Gravitational energy is being converted to rotational kinetic energy D) Energy comes from cookies. E ) I have no clue….

Turning the bike wheel A student sits on a barstool holding a bike wheel. The wheel is initially spinning CCW in the horizontal plane (as viewed from above) L= 25 kg m 2 /s She now turns the bike wheel over. What happens? A. She starts to spin CCW. B. She starts to spin CW. C. Nothing Start w/ angular momentum L pointing up from wheel. When wheel is flipped, no more angular momentum from it pointing up, so need to spin person/stool to conserve L! CORRECT

Turning the bike wheel (more) She is holding the bike wheel and spinning counter clockwise. What happens if she turns it the other ½ rotation (so it is basically upside down from how it started). A) Spins Faster B) Stays same C) Stops

Turning the bike wheel... Since there is no net external torque acting on the student-stool system, angular momentum is conserved. Remenber, L has a direction as well as a magnitude ! Initially: L INI = L W,I = + 25 kg m 2 /s Finally: L FIN = L W,F + L S = -25 kg m 2 /s + L s L s = 50 kg m 2 /s L W,F L S L W,I L W,I = L W,F + L S

Act 2 Rotations  A puck slides in a circular path on a horizontal frictionless table. It is held at a constant radius by a string threaded through a frictionless hole at the center of the table. If you pull on the string such that the radius decreases by a factor of 2 , by what factor does the angular velocity of the puck increase? (a) 2 (b) 4 (c) 8 Puck on ice

Act 2 Solution Since the string is pulled through a hole at the center of rotation, there is no torque: Angular momentum is conserved. L 1 = I 1  1 = mR 2  1    m R    m R/2 L 2 = I 2  2 = m  2 = mR 2  1 = m R 2  2  1 =  2  2 = 4  1

Gyroscopic Motion: Suppose you have a spinning gyroscope in the configuration shown below: If the left support is removed, what will happen??  pivot support g

Gyroscopic Motion... Suppose you have a spinning gyroscope in the configuration shown below: If the left support is removed, what will happen? The gyroscope does not fall down!  pivot g

Gyroscopic Motion... ... instead it precesses around its pivot axis !  pivot Bicycle wheel

Summary  = I  L = I  Right Hand Rule gives direction If  = 0, L is conserved

Lecture16

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